1. Field of the Invention
The present invention relates a real-image finder optical system.
2. Description of the Related Art
In a lens-shutter type camera, a real-image finder optical system, which is provided independently from the photographing optical system, generally includes a positive objective optical system, an erecting optical system, and a positive eyepiece optical system. In the real-image finder optical system, the objective optical system forms an inverted image which is upside down and reversed from left to right, and the erecting optical system erects the inverted image to the proper orientation. The erected image is viewed through the eyepiece optical system. In such an erecting optical system, a Porro prism having four reflection surfaces is generally used. Further, an erecting optical system is usually provided between the inverted image formed by the objective optical system and the eyepiece optical system.
In the above explained prior art, since an erecting optical system is provided between the inverted image formed by the objective optical system and the eyepiece optical system, the optical path from the inverted image to the eyepiece optical system is long. Therefore the focal length of the eyepiece optical system has to be made longer. However, if the focal length of the eyepiece optical system is long, the finder magnification (f.sub.o /f.sub.E ; f.sub.o : the focal length of the objective optical system; f.sub.E : the focal length of the eyepiece optical system) becomes lower, which results in a decrease of the apparent visual angle. Due to this decreased the apparent visual angle, an operator feels difficulties in observing an image through the eyepiece optical system, and further the quality the image is deteriorated.
The size of the eyepiece optical system in the radial direction is substantially determined by the apparent visual angle and the eye relief. More specifically, the larger the apparent visual angle and the eye relief become, the larger the eyepiece optical system is in the radial direction. Furthermore, the size of the light emitting surface of the erecting optical system is substantially the same as the final surface of the eyepiece optical system. In addition, it is understood that the optical path length of the erecting optical system is approximately three times as long as the diameter of the light emitting surface of the erecting optical system. Therefore if the eye relief and the apparent visual angle are made larger, the optical path length of the erecting optical system is inevitably made longer, and thereby the finder magnification decreases as explained. Subsequently, the erecting optical system is made larger, and finally the entire finder optical system has to be made larger.